U.S. patent number 4,183,065 [Application Number 05/951,424] was granted by the patent office on 1980-01-08 for video recording apparatus which samples and quantizes low frequencies and then groups and records an analog representation thereof.
This patent grant is currently assigned to Eastman Kodak Company. Invention is credited to Edward M. Granger, Frederic H. Metildi, Harvey E. Rhody.
United States Patent |
4,183,065 |
Rhody , et al. |
January 8, 1980 |
Video recording apparatus which samples and quantizes low
frequencies and then groups and records an analog representation
thereof
Abstract
Apparatus for directly recording television signals on a
magnetic medium strips out sync and blanking information. Low
frequency components of the television signals are time compressed,
i.e. frequency shifted, and recorded as a substitute for the
stripped-out sync and blanking information. Thus, high frequency
components of each video line are recorded, followed by that line's
low frequency components, thereby facilitating the playback of low
frequency information. In time compressing the low frequency
information, the video signal content of a line is sampled at a
predetermined rate. Each sample is then quantized for purposes of
improving the playback signal-to-noise ratio, i.e. the effect of
tape modulation, and other, noise is nullified by sample
quantization. And, to cut the bit rate resulting from sample
quantization, a preferred form of the invention groups the
quantized bits and records analog representations of the bit
groups.
Inventors: |
Rhody; Harvey E. (Fairport,
NY), Metildi; Frederic H. (Penfield, NY), Granger; Edward
M. (Rochester, NY) |
Assignee: |
Eastman Kodak Company
(Rochester, NY)
|
Family
ID: |
27100223 |
Appl.
No.: |
05/951,424 |
Filed: |
October 16, 1978 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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669923 |
Mar 24, 1976 |
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Current U.S.
Class: |
386/328; 360/24;
360/27; 386/269; 386/306; 386/327; 386/E5.009 |
Current CPC
Class: |
H04N
5/92 (20130101) |
Current International
Class: |
H04N
5/92 (20060101); H04N 005/79 (); H04N
005/795 () |
Field of
Search: |
;360/8-10,24,27,32-33,36-37 ;358/4,12,127,133,138,141,142,145-148
;179/15.55T |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1242673 |
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Jun 1967 |
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DE |
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1263071 |
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Mar 1968 |
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DE |
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Primary Examiner: Cardillo, Jr.; Raymond F.
Attorney, Agent or Firm: Cody; Robert F.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The following applications are related to each other:
This is a continuation of application Ser. No. 669,923, filed Mar.
24, 1976 now abandoned.
Claims
What is claimed is:
1. Apparatus for recording a television signal comprising:
(a) means for dividing said signal into high and low frequency
bands;
(b) means for receiving said low frequency band for time
compressing said low frequency band;
(c) means for recording on a magnetic medium first said high
frequency band and then said time compressed low frequency
band;
said means for time compressing said low frequency band
comprising:
(a) means for sampling said low frequency band at a predetermined
rate;
(b) means for quantizing said signals;
(c) means for grouping the bits which comprise said quantized
samples;
(d) means for converting said grouped bits to analog
representations of said grouped bits; and
(e) means for applying said analog representations of said grouped
bits to said magnetic medium.
2. Signal processing apparatus for use in recording a television
signal comprising:
(a) high and low pass filter means adapted to receive said
signal;
(b) means for sampling the output of said low pass filter
means;
(c) means for quantizing the sampled output of said low pass filter
means;
(d) means for grouping the bits which comprise said quantized
samples;
(e) means for converting said grouped bits to analog
representations thereof;
(f) means for recording on a magnetic medium the signal output of
said high pass filter means followed by the analog representations
of said grouped bits;
(g) means for playing back signal information from a magnetic
medium;
(h) means for re-converting analog representations of said grouped
bits to their quantized equivalents;
(i) means for forming analog signal samples from the bit output of
said means for re-converting; and
(j) means for combining played back high pass signal information
with the analog signal output of said means for re-converting.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to video recorders, especially those which
employ magnetic tape as the recording medium, and which, on
playback, feed video signals, say, to the input terminals of a
television set. The term "recorder" as used herein shall be
interpreted to include playback apparatus as well.
2. Description Relative to the Prior Art
As is known, the video spectrum, as typified by the NTSC baseband
signal, contains frequency components from DC to 3 MHz or more, the
lower frequencies corresponding at least in part to scene
background information, and the higher frequencies corresponding to
scene detail information. Because the playback of information
recorded on a magnetic medium such as magnetic tape is
frequency-dependent (i.e. system response to the playback signal is
dependent on the rate of change of flux as seen by a playback
head), flux patterns which correspond to low frequency signal
components are not easy to detect in the magnetic tape. Prior
attempts to record and play back, say, the baseband NTSC signal
have, therefore, usually involved the baseband-modulation, during
recording, of a relatively high frequency carrier. By so using a
baseband-modulated carrier, the playback circuits of the recorder
can be tuned to respond to the rapidly changing flux
characteristics which correspond to the recorded carrier, with the
baseband video signal being then relatively easy to detect and
remove from the modulated carrier. While the practice of recording
a modulated carrier solves the problem associated with the playback
of low frequency information, it does so not without cost: First,
because a high frequency carrier must be recovered during playback,
the gap in the playback head--for a given speed of tape past the
playback head--must be narrower than for the case of a directly
recorded baseband signal, a factor which usually means higher head
cost. And, second, because the baseband signal, when it modulates
the carrier during the recording operation, produces signal
sidebands--which must be recorded, and recovered during
playback--the spectrum of frequencies to be processed by the
recorder (i.e. the recorder bandpass requirements) will be wider
than in the case of a baseband recorder. Such bandwidth
requirements directly affect information packing density; and,
thus, for maximum tape economy for a given amount of video
information, the prior practice of recording a modulated carrier
leaves much to be desired.
As indicated above, playback of magnetically recorded information
depends on the rate of change of flux as seen by a magnetic head.
This may suggest that--rather than place the baseband video signal
on a carrier as a vehicle for playing back low frequency video--the
track containing the recorded video information be widened a bit
for cooperation with a widened magnetic playback head, the recorded
information being the full video baseband signal. In that way,
flux-to-head linkage will increase and, attendantly, so will the
playback signal-to-noise ratio (SNR)--which should improve the
recoverability of low frequency information. But widening the
record track works counter to the goal of "high-packing-density
tape usage". Besides, low frequencies, when recorded on a magnetic
medium, create flux patterns within the medium which tend to spread
out and, to assure that the low frequencies recorded in one track
are not played back during the playback of an adjacent track,
sizable guard bands between the tracks have to be used, again
adversely influencing the efficient use of the magnetic medium.
Aside from the above techniques for recovery of low frequency
information from a magnetic recording medium, one might also
consider separating the low frequency components of the baseband
signal from the higher frequency components, and then recording the
low frequencies on a carrier in a separate track. But the use of
two tracks to record the baseband information obviously is not in
the interest of tape efficiency.
In copending application Ser. No. 669,925, filed Mar. 24, 1976 in
the name of Edward M. Granger, now U.S. Pat. No. 4,104,683 baseband
recording of the NTSC signal is indicated as being implemented by
splitting the baseband signal into high and low frequency bands,
the low frequency band being then time compressed and recorded
analog-wise as a substitute for the sync and blanking information
contained in the NTSC signal. Such a technique will work well so
long as tape and other noise is relatively low, for low frequency
SNR is extremely critical in the faithful reproduction of recorded
scene information.
SUMMARY OF THE INVENTION
To immunize, from noise, the baseband recording of time compressed
low frequency information which is substituted for sync and
blanking information during the recording process, the present
invention proposes, prior to recording, the quantization of such
low frequency information; and, to cut the quantization bit rate
which would have to be handled during the sync and blanking period,
the invention further proposes the grouping, according to a
predetermined sequence, of the quantization bits, followed by a
reconversion and recording of the grouped bits as analog
signals.
The invention will be described with reference to the figures, of
which:
FIG. 1 is a series of diagrams useful in explaining the
invention;
FIG. 2 is a block diagram of one embodiment of the invention;
and
FIG. 3 is a block diagram of another embodiment of the
invention.
As indicated above, low frequency SNR is critical to the high
quality display of recorded video information. To enable a
relaxation in the noise figure for the Granger recorder (U.S.
application Ser. No. 669,925), and its tape, the invention proposes
quantization of the low frequency information which gets
substituted for sync and blanking information. How and why
quantization relaxes the low frequency noise requirements of the
Granger recorder may be appreciated from a consideration of FIG. 1:
Consider the low frequency signal sample S of FIG. 1A which is to
be recorded on magnetic tape. Assuming tape noise as depicted in
FIG. 1B, it is apparent that, during playback of the Granger
recorder, close reproduction, as is so necessary, of the recorded
low frequency sample may be difficult. By quantizing the low
frequency sample (FIG. 1C), the maximum error that the playback
signal will experience (assuming no loss of bit information) will
be the relatively small error e.sub.Q. (The error e.sub.Q will vary
directly according to the number of quantization bits employed.)
Rather than require a playback system to discern signal amplitudes
(as in FIG. 1B), FIG. 1D, which is representative of a principal
teaching of the invention, indicates that, even in the presence of
noise, the presence or absence of a quantization bit is relatively
easy to determine; and thus low frequency noise can be effectively
filtered away, during playback, by quantizing the low frequency
samples.
Referring to FIG. 2, a video recorder, embracing both the teaching
of U.S. application Ser. No. 669,925 and the teaching of the
present invention, receives a television signal off an antenna 10
and detects the information or baseband part of that signal by
means of a detector 12. A circuit 14, for removing the sync
information contained in the output of the baseband detector 12,
applies its horizontal sync output h to a circuit 16 adapted to
produce a signal corresponding to the duration of the blanking
signal on which the sync signal rides. The blanking detector 16,
typically, will include a counter, a particular count of which is
kept in sync with an edge of the sync pulse, and which counter is
reset when its count reaches a count corresponding to the duration
of the blanking period. The vertical sync pulse output v from the
sync separator 14 is applied via a RECORD-PLAYBACK switch to a
magnetic head 22 for laying down a control track on magnetic tape
24. In addition, the vertical sync pulse v is applied to a servo 26
which slaves the speed of a head wheel 28 to the vertical sync
pulse rate.
The video information contained in the output of the baseband
detector 12, as taught in U.S. application Ser. No. 669,925, is
applied via an electronic switch 30 to high and low pass filters
32, 34, respectively, the switch 30 being actuated (opened) at the
horizontal sync rate by the output of the blanking detector 16. The
high frequency content of the baseband signal is applied via an
electronic switch 36, and slip rings 38, to magnetic heads 40, 42
which cooperate with and scan the tape 24 in a well-known
manner.
At the same time that the high frequency content of the baseband
signal is recorded, the low frequency content of the baseband
signal is processed for recording as a replacement for the sync and
blanking information eliminated by actuation of the switch 30.
Briefly, the low frequency output of the filter 34 is sampled at a
first rate and quantized; then, during a period that corresponds
with the horizontal blanking portion of the baseband signal, the
quantized low frequency samples are clocked at a relatively high
rate onto the tape 24. Assuming the low frequency band extends from
DC to 40 KHz, in order to meet the Nyquist criterion and thus avoid
aliasing effects, the sampling of low frequency information must be
such that the number of samples N taken across a horizontal line of
low frequency video information times the horizontal line rate
equals at least twice the upper frequency edge (i.e. two times 40
KHz equals 80 KHz) of the low frequency band. And, in this regard,
seven samples of low frequency information have been found
adequate, an oscillator 44 being operated at seven times the line
rate of 15.75 KHz to effect such sampling by a sample-and-hold
circuit 46. The sampling oscillator 44 applies its sampling signals
to the sample-and-hold circuit 46 via an electronic switch 48
during the occurence of each line of low frequency information. As
soon as a low frequency sample is taken, it is immediately
quantized (six bits) by an analog-to-digital converter 47 (A/D) and
loaded, in bit form, into a register 49 by means of an oscillator
51 and switch 53. Then, the next sample is similarly taken,
quantized, and loaded into the register 49 as the first sample
progresses down the register, etc. After the seventh sample is
taken, and quantized, the blanking signal occurs, opening the
switches 48 and 53, and closing a switch 50. As the switch 50
closes, an oscillator 54 clocks the 42 bits of low frequency
information to the tape 24. Since the blanking signal has a
duration which is about one-fifth the duration of a horizontal scan
line, and since each sample is represented by q bits (where q
equals six), the oscillator 54 has a frequency at least about 5q
times that of the oscillator 44. In other words, while low
frequency samples are slowly taken over the course of a horizontal
line, they--as represented by 42 bits within the register 49--are
quickly clocked onto the tape during the relatively brief blanking
period, and thus appear as an easily recoverable high frequency bit
rate, at least as far as head-to-tape interaction is concerned.
During playback, reverse signal processing is employed: A servo 60
(which obviously may be the servo 26 operated through suitable
switching) drives the head wheel 28 under control of the control
track head 22; and vertical and horizontal sync pulses are produced
by a sync pulse generator 62 responsive to the control track
information (head 22), the sync pulses being applied to a summing
circuit 64 adapted to reconstruct the baseband signal. A blanking
detector 66 like the detector 16 (or, with suitable switching, the
same one) produces switching signals for use in gating the playback
information to either of two processing channels. The playback
information--comprising, in sequence, a band of high frequency
signals occurring over a 1-h duration, followed by 42 bits of low
frequency information occurring over a "blanking" duration,
followed by a band of high frequency signals occurring over a 1-h
duration, followed by 42 bits of low frequency information
occurring over a "blanking" duration, etc.--is applied via slip
rings 38, and playback amplifier 70, to an electronic switch 72.
The switch 72 is actuated by the playback blanking signal (circuit
66) and steers the playback information as follows: the high
frequency information is applied to a 1-h delay 74 which lets the
low frequency information--corresponding to such high frequency
information, and recorded timewise on the tape after the high
frequency information--to catch up in time with the high frequency
information; and the low frequency information, which was quantized
and time compressed for recording during the blanking period, is,
during the playback blanking period, loaded quickly into a register
75 by use of the high frequency oscillator 54 (switch 84). Then, by
use of the oscillator 51, and switch 53, the register 75 is
unloaded as the high frequency information progressively appears at
the output of the 1-h delay 74. As the bits corresponding to the
successive low frequency samples are clocked out of the register
75, they are converted to analog form (D/A 77), and held in a
sample-and-hold circuit 78--the circuit 78 under control of the
oscillator 44, and switch 86, applying its samples to the summing
circuit 64 in proper phase with the high frequency signal
information with which such samples correlate. With the high and
low frequency information, and the sync and blanking information,
all appearing at the summing circuit 64 in proper time, the circuit
64 produces a baseband signal which may, for example, be modulated
on a carrier and played through a television receiver.
Not only to stabilize system timing, but also to avoid introducing
unwanted modulation of the processed low frequency information, the
oscillators 44, 51, and 54 are all phase-locked to sync
information, either the sync information derived off the air, or
the sync information derived off the tape.
To be realized is that in the apparatus as described above,
bits--and not signal amplitudes--are processed while practicing the
invention, "bit-rate variation" being the vehicle for time
compressing the low frequency information, whereby such information
may be used as a substitute for sync and blanking information.
Since bits can be saturation-recorded, their occurrence, or not, is
all that has to be detected, during playback, thus effectively
nullifying the influence of low frequency noise.
In practicing the invention as taught in connection with the
apparatus of FIG. 2, the bit rate--assuming seven six-bit samples
for each horizontal line of video--will be approximately 3.5
megabits per second. Such a bit rate may, depending on system
design, be too high to process effectively. By means of the
invention in its presently preferred form, the bit rate is
effectively reduced by a bit-grouping technique, typically by
pairing bits and converting each bit-pair to one of four
representative analog levels. The analog levels are then recorded;
and during playback, such levels are decoded into bit-pairs,
converted to six-bit groups, and then each six-bit grouping is
converted to one of seven low frequency analog samples.
FIG. 3 shows a system employing the concepts embodied in the
apparatus of FIG. 2 as modified by the bit-grouping technique: Save
for the addition of certain signal processing hardware, the system
of FIG. 3 is largely the same as the system of FIG. 2. Thus, the
description which follows will only address the FIG. 2 components
as they relate to the bit-grouping concept:
During recording, as sample bits emerge at a high rate from the
register 49 in response to a blanking signal applied to the switch
50 (the blanking signal allowing the high frequency oscillator 54
to apply its output to the register 49) they (i.e. the bits) are
applied serially to a two-bit register 100 which serves as a
"bit-grouper". After each successive pair of bits is briefly stored
in the register 100, it (i.e. the bit-pair) is gated (102) to a
digital-to-analog converter 104 (D/A). Such gating is under control
of a clock that is one-half the frequency of the oscillator 54
output, a frequency divider 106 (the divisor g of which is "two" in
the case at hand) providing the gating signal which is applied to
the gate 102. The D/A converter 104 output, which appears as a
four-level analog signal, is then applied (either on a carrier or
not) as a substitute for sync and blanking information, via the
switch 36, to and recorded on the tape 54. Thus, while digital
processing is employed to modify the effect of noise on low
frequency sample information to be recorded as replacement for sync
and blanking information, the substitute signal information which
is actually recorded, as in FIG. 3, appears as a signal
corresponding to a bit-rate that is only one-half the bit-rate
employed in connection with the apparatus of FIG. 2.
During playback, some smoothing of the four-level analog signal
occurs. See the playback signal appearing at the input to the
amplifier 70. In response to the playback blanking signal (66), the
four-level analog signal is applied via the switch 72 to an
analog-to-digital converter 110 (A/D), the digital output of which
is gated (112), parallel-wise, by the frequency divider 106 output
into a two-bit register 114. At twice the frequency of the
frequency divider 106 output, i.e. in response to the high
frequency oscillator 54 output applied via a switch 116, the
contents of the register 114 are serially loaded into the register
75. During the playback blanking period, the low frequency
information, in digital form, is stored in the register 75, and
readied for conversion to analog form for complementing the high
frequency video information as it appears at the output of the
delay 74. To this end, a six-bit digital-to-analog converter 120
(D/A) receives the register 75 output as it is relatively slowly
clocked out (switch 53) during the non-blanking part of each line
of playback video information. As the last bit in each succession
of six bits is applied to the D/A converter 120, a sample-and-hold
circuit 122, cooperative with the D/A converter 120, has its
six-bit "held" signal clocked (the switch 86 being closed) to the
summing circuit 64 in proper phase with the high frequency
information applied to the switch 64 by the delay 74.
The invention has been described in detail with particular
reference to preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
* * * * *